Direct Synthesis of Highly Substituted Cyclopentadienes and Derivatives from the Self‐Condensation of Renewable Ethyl Levulinate.

Tri‐substituted cyclopentadienes and cyclopentanes, high‐value‐added compounds, are readily prepared from ethyl levulinate, a chemical platform easily obtained from biomass. Key steps involve coupling two molecules of ethyl levulinate in the presence of pyrrolidine, followed by a base‐catalyzed intramolecular aldol condensation, to provide a singly‐purified and easily derivatizable cyclopentadiene in almost 40 % isolated overall yield. Reaction conditions are mild, simple and employ green solvents such as ethanol and anisole. This method opens a new pathway to preparing complex molecules from renewable feedstocks. The synthesis of three‐substituted cyclopentadienes and cyclopentanes is achieved from renewable ethyl levulinate (EL), an important biomass platform. Using enamine chemistry, EL is promptly converted to a substituted cyclopentadiene after condensation and cyclization. Derivatization of this intermediate provided several cyclopentadienes and cyclopentanes in just one step. [ABSTRACT FROM AUTHOR]